The present invention relates generally to a wave plate retarder, and is directed more particularly to a 1/2 wavelength plate or 1/4 wavelength plate used, for example, in a reproducing (pick-up) apparatus of an optical video disc.
In a reproducing apparatus for an optical video disc, an apparatus is known in which light from a laser is impinged on a signal track recorded on the video disc. The reflected laser light therefrom, which is modulated by recorded pits in the signal track, is detected, and the recorded signal is read out. In order that the laser light reflected from the video disc is effectively introduced to its detector and to avoid the reflected laser light returning to its source and causing noise, a combination of a 1/4 wavelength plate and a polarizing beam splitter is used to effectively separate the laser light reflected from the video disc from the incoming light path.
Any suitable birefringent material such as, for example, mica, rock crystal or high molecular film may be used to make a 1/4 or 1/2 wavelength plate. Unfortunately, mica and rock crystal are rather expensive and, while high molecular film is relatively inexpensive, it normally has unstable characteristics. High molecular films, such as for example, polypropylene film which has been elongated by being stretched during manufacture on an elongation roll, has an optical axis which is aligned with the longitudinal direction of stretch of the film. Such a high molecular film has birefringent or double-refractive properties. A material having birefringent properties has different refractive indexes and light transmission speeds for light polarized in different, usually orthogonal, directions. When a beam of light is passed through a birefringent material normal to its optical axis, the birefringent material breaks up the light into an ordinary ray and an extraordinary ray which travel through the material at different speeds. It is a further property of birefringent material that the oridinary ray and the extraordinary ray are linearly polarized in mutually perpendicular directions. If the thickness of a high molecular film can be selected such that one of the rays passing through the film is retarded by a 1/4 or 1/2 wavelength relative to the other ray, the resulting phase difference (retardation .GAMMA.) between the two rays is 90.degree. or 180.degree. respectively.
In practice, however, it is difficult to control the thickness of stretched high molecular films accurately enough to use them for 1/4 wavelength or 1/2 wavelength plates. Even if the thickness could be controlled with sufficient accuracy, the birefringent properties of the material are not uniform from lot to lot due to unavoidable differences in composition and manufacturing conditions between lots. It is therefore so difficult to select or to manufacture high molecular films having a desired phase difference .GAMMA. for a particular wavelength of light that such high molecular films are not widely used and cannot be manufactured inexpensively.